<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        #demo{
            width: 100vw;
            height: 100vh;
            background: brown;
            display: flex;
            align-items: stretch;
            justify-content: space-between
        }
        .div1,.div3{
            width: 100px;
            flex: none;
            background: #fff
        }
        .div2{
            flex: auto;
            background: #000
        }
    </style>
</head>
<body>
    <div id="demo">
        <div class="div1"></div>
        <div class="div2"></div>
        <div class="div3"></div>
    </div>
</body>
</html>